Minimum is 2.5 inches from top of rail to bottom of freight car per 49 CFR 215.121(a). The height of the rails and the tie plates could have an impact on the actual clearance and height of the train. If you have the crazy idea to plop down on the tracks and let one go over you, you risk being clobbered by loose chains, other debris, getting scared and lifting your head, etc. Pretty much a dumb idea all the way around!